Friday, July 30, 2010

PDMS

The final goal of our diffraction grating lab was to make something we could actually use as a grating. Our silicon wafer isn't transparent though, so we can't shine light through it! We need to make something transparent, with the same pattern as wheat was on the wafter. Here is out finished wafer. You can see the printed pattern in it:

PDMS stands for Poly-dimethyl-siloxane, which is a polymer made out of silicon, oxygen, carbon, and hydrogen. If you have ever seen silicone (what's in breast implants, cookware, some bike seats, and a myriad of different uses) this is a very similar substance, and sets as a clear gel. So we're basically putting sili-CONE on sili-CON. When I hammer on how important using the right words are, this is what I mean. One is a soft squishy polymer, the other is a shiny brittle crystaline substance. Here we are coating the silicon wafer in the PDMS:

Once the PDMS sets, it should mold to the same pattern as is on the silicon, except in reverse, just like any other mold:

The final part is de-bubbling and baking the PDMS so it sets. Inside this oven is a vacuum, so any air bubbles inside the PDMS suddenly have a much higher pressure than what is in the oven. They will expand and bubble out to the surface. Remember gas laws! The bubbles are not sucked out,t hey push themselves because there is no longer any equal pressure holding them down! (imagine pushing against someone, and then they move out of the way!) Here is the PDMS bubbling out all the air in the vacuum oven:

They need to stay in there for close to two hours, so we left them in the capable hands of engineer who was walking us through the process.